net: mpls: Fix setting ttl_propagate for rt2

Fix copy and paste error setting rt_ttl_propagate.

Fixes: 5b441ac8

 ("mpls: allow TTL propagation to IP packets to be configured")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>